feat: pagination component (Web Awesome parity)

[eddyloewen]

Goal

Add pagination to the library, following the Web Awesome anatomy of wa-pagination (attributes, slots, CSS parts, events) but implemented with element-js on the shared style layer from the "style layer" issue in this milestone.

Page navigation: page, total, per-page, sibling/boundary counts with ellipsis, prev/next, nav landmark with aria-current, change event.

Acceptance criteria

• src/components/pagination/ with the standard files: pagination.js (class + define()), pagination.define.js, pagination.style.js (composed from the shared style modules), pagination.events.js where events exist, pagination.stories.js, pagination.readme.md, pagination.changelog.md, pagination.test.unit.js, pagination.test.feature.js.
• Attributes reflect to properties; sizes small/medium/large and the variant vocabulary of the style layer where they apply.
• Accessible: correct roles/ARIA, keyboard operable, focus visible; axe passes in the feature test.
• Renders server-side through @webtides/element-js-ssr-renderer (catalog smoke test passes) and hydrates without a flash.
• Custom-elements manifest, catalog, all.js, and README component list updated.

Depends on

Style layer issue of this milestone.

[eddyloewen]

Depends on the style layer: #69